AND THE SEXUAL DIFFERENCES OF THE EYES IN DIPTERA. 3" 



(4) Eyes situated on Lateral Projections. 



This character must afford the possessors of it a greater range 

 of view — the whole horizon, in fact, and probably some descrip- 

 tion of stereoscopic vision in front. It seems to have been 

 brought about by sexual causes, as we find the female much less 

 developed in the spread of the projections (but, I think, equally 

 ■developed in the size of the eyes). Aly observations on this point, 

 however, are much too few to speak with any certainty. 



(5) The Pubescence on the Eyes. 



The advantages and disadvantages of this character are, I con- 

 fess, very difficult to understand. To try to solve the problem 

 I made the following experiments : 



Experiment 1. — A number of round holes about 1 mm. in 

 diameter were made in a card, so that the centre of each circle 

 was 3 mm. from the centre of the surrounding circles. 



(a) This was placed at various distances from the eye. Very 

 little obstruction to vision was met with, and the outline and 

 details of objects about eighty yards off were clearly seen. 



(b) A strong light was screened, and a small hole was made in 

 the screen, on which the card was placed. The light from the 

 holes was focused on a white' surface, and faintly illuminated a 

 much larger space than the 16-mm. square experimented with. 

 The spaces opposite the holes were brightly illuminated, and 

 the areas between the holes were brighter than the surrounding 

 larger space described above. 



Experiment 2. — A similar card was prepared with holes at 

 the same distances. Between these holes inch pins of the usual 

 thickness (not entomological) were inserted, and Experiment 1 (a) 

 was repeated with surprising results, as the view was scarcely 

 if at all interfered with. Experiment 1 (6) was also repeated, 

 the pins turned to the light, and their bright surfaces reflected 

 light, but again no appreciable advantage or disadvantage was 

 experienced. 



